The invention relates to microcontroller-controlled electronic apparatuses, and more specifically to a microcontroller-controlled electronic apparatus utilizing a low voltage reset circuit to protect the microcontroller from operating in an indeterminate state.
There are a variety of known electronic apparatuses that utilize a microcontroller, e.g., a microprocessor, to control one or more associated current-consuming devices that are part of the apparatus. Power is supplied to the microprocessor in such an apparatus from a supply voltage source (designated herein as "VCC") to a voltage supply input pin on the microprocessor (designated herein as "VDD").
If the supply voltage fall's below the microprocessor's minimum acceptable level, the microprocessor will no longer operate properly. This minimum voltage level is the microprocessor manufacturer's minimum operating voltage specification for the microprocessor. When power to the microprocessor falls below this specified minimum operating voltage level, the microprocessor enters an indeterminate state (also known as a lock-up state or a brown-out state). When the microprocessor is in the indeterminate state, it will cease proper execution of its associated software program and predictable program control will no longer exist in the system. As a result, it is no longer possible to effectuate a definite control of the associated devices in the apparatus that the microprocessor otherwise properly controls when in its normal, determinate state.
To solve this problem, manufacturers of many of the more expensive microprocessors provide brown-out protection circuitry internal to the microprocessor chip that measures and monitors the supply voltage (VCC) and operates to shut down or reset the microprocessor to protect it against malfunction in the event of the supply voltage falling below the minimum operating level. A problem with many of these supply voltage level-detection arrangements is that transients in the supply voltage can lead to false resets of the microprocessor. This often results in a malfunctioning electronic apparatus. Further, the added electronic components needed to measure and monitor the supply voltage results in an increase in the cost of the overall integrated circuit. An example of such a microprocessor with brownout detection is disclosed in U.S. Pat. No. 5,606,511 to Yach. The Yach arrangement recognizes the false reset problem of supply voltage level detection arrangements and provides a discriminator circuit to distinguish between supply transients and an actual brown-out situation. The addition of this discriminator circuit contributes further to the design complexity and associated costs of the overall integrated circuit.
The field of microprocessor-controlled electronic apparatuses has expanded beyond the traditional uses in large appliances, industrial control applications and automotive arrangements, as discussed in the Yach patent. Those conventional uses generally involve systems having a controlled power supply. The expansion of the field has led to an increasing use of microprocessor-control arrangements in much smaller, portable devices, such as toys, that are battery-controlled. In these portable devices, a microprocessor lock-up situation can leave the device in a state in which power consuming components continue to draw current from the system batteries. Accordingly, the indeterminate state scenario may substantially reduce the product's battery life. The problem of excessive current draw in these portable battery-powered products generates a higher concern than in other arrangements such as AC-powered products because of the increased importance of conserving power in a battery-powered product.
Moreover, when the electronic apparatus is in an indeterminate state, current draw from the associated microprocessor-controlled devices may continue even though the series-connected battery cells in the supply voltage source are nearly depleted. This situation can result when the microprocessor-controlled associated devices were not properly set to an off-state because of the loss of predictable program control in the indeterminate state. As a result, the draining of the voltage supply's battery cells down to low voltage levels below their recommended cut off thresholds can lead to cell reversal and short circuits. In cell reversal, as the charge in a particular cell approaches zero volts, the positive and negative designations may switch to an opposite orientation. As a result, other battery cells in the voltage supply of the apparatus may begin to charge the lowest voltage cell and place energy into it due to the series arrangement of the battery cells within the power supply. Such a situation can lead to undesired and possibly dangerous electrolyte leakage in the portable battery operated products.
Another problem that exists when portable battery-powered microprocessor-controlled products enter an indeterminate state is that the loss of hardware control will often leave some of the microprocessor-controlled associated devices in an ON-state, while others will be turned OFF. This has the further disadvantage of giving the consumer the impression that the apparatus is defective because the entire apparatus is not operational.
In the field of portable, battery-powered microprocessor-controlled products, it is usually not cost-effective to use the more expensive microprocessors described above that have internal brown-out protection circuits. These high-end microprocessors often involve complex circuitry and design and are thus more costly. Thus, many industries manufacturing portable microprocessor-controlled products, such as the toy industry, which produces many battery-operated portable electronic products, put an emphasis on low production costs and simplicity in any necessary electronic circuit designs. The toy industry therefore typically utilizes inexpensive microprocessors in its products. However, these inexpensive microprocessors have no means for shutting down in a predictable manner when a low voltage situation causes the microprocessor to stop execution and enter an indeterminate state. As a result, the associated devices to be controlled by the microprocessor in the portable apparatus could remain in an undesirable state, resulting in the above-described problems that result in battery-powered devices.
It would therefore be desirable to provide a cost-effective apparatus arrangement and method to reset a microprocessor in a portable apparatus to a known state after a low supply voltage has caused the microprocessor to enter an indeterminate state.
It would also be desirable to provide a method for use with battery-powered products having multiple associated devices that will simultaneously shut down all of the devices at the end of battery life to prevent continued current draw as well as consumer perception of a defective product if the device is only partially functioning.
Finally, it would be desirable to provide a reset arrangement and method that also eliminates problems inherent in the prior art supply voltage level detection arrangements such as design complexity and high manufacturing costs, as well as supply voltage transients leading to false system resets.